759
FXUS66 KPQR 242221
AFDPQR
Area Forecast Discussion
National Weather Service Portland OR
321 PM PDT Wed Apr 24 2024
.SYNOPSIS...Our stretch of drier weather finally comes to an
end - wetter April weather had to return at some point. A
pacific frontal system is slated to arrive Thursday morning
bringing widespread rainfall to the region and heralding the
start cooler, showery, and unsettled conditions through the
weekend into next week. Its worth noting Friday into Saturday
snow will also once again be possible down to the highest
Cascade passes above ~4,500ft, however, any appreciable snowfall
impacts likely remain confined to the mountain tops above
5,500ft.
&&
.SHORT TERM...Tonight through Sunday...This afternoon and
evening we continue to wait on the arrival of our next weather
disturbance in the form of any upper-level trough and
accompanying frontal boundary currently moving over the eastern
Pacific towards the Pacific Northwest. Latest high resolution
guidance like the HREF has pushed back the arrival of this
feature by an hour or two late tonight into Thursday morning but
in general expect rainfall to reach the I-5 corridor by 4-6am
with periods of steady light rain through the day before
breaking to showers Thursday evening. The upper-level forcing
associated with this disturbance leaves much to be desired but
itll at least come with a decent moisture plume highlighted my
model PWAT values pushing 0.8-1 which is around 0.3 inches
above normal. Thursday night into Friday morning the axis of the
aforementioned trough moves overhead pushing snow levels near
4,500-5,000ft. While some wet snow likely makes it down to the
higher pass areas like Santiam and Willamette pass, the warmer
antecedent conditions should help to limit any pavement
accumulation before daytime heating takes hold later Friday
morning. This daytime heating and cooler temperatures aloft will
allow for some additional instability come Friday afternoon
which could support locally moderate to heavy showers, although
after interrogating deterministic model soundings from the GFS,
ECMWF, and NAM the available CAPE doesnt appear deep enough to
support T-storms and is held below the mixed phased(lightning
generation) portion of the atmosphere(-10 to -20c). As far as
precipitation totals are concerned Thursday through Friday, the
NBM projects rainfall amounts of 0.6-0.9 inches across the
Willamette Valley/SW Washington, and 1-2.5 inches for the
coast/coast range and Cascades - locally the highest along the
coast range in Tillamook/Clatsop County.
Late Friday into Saturday the upper-level trough feature
amplifies and quickly progress southeastward into the four
corners region of the Great Basin while heights briefly rise
over the region due to the passage of a weak transient shortwave
ridge. Confidence is high this will help to consolidate
lingering shower activity to primarily the elevated terrain of
the coast range and Cascades leaving the valley with 10-25%
PoPs. That said, guidance indicates the arrival for an upper-
level shortwave and weak warm-frontal boundary extending
southward off a parent low parked over the gulf of Alaska
increases chances for precipitation Saturday afternoon through
Sunday. Temperatures late week through the weekend remain near
to slightly below normal for mid to late April. /Schuldt
.LONG TERM...Sunday Night through Wednesday...Pattern is rather
progressive, with fronts moving off the northeast Pacific into
the Pacific NW. Yet another front will push across the region
Sunday night, with post-frontal showers on tap for Monday. Still
a bit cooler than that expected for late April, with overall
temperatures in the lowlands in the 50s to lower 60s. Snow
levels will run 2500 to 3500 ft, which will maintain some snow
at times for the higher terrain of the foothills, as well as
most of the Cascades.
Broad upper low over the southeast Gulf of Alaska will gradually
drop southeastward off the Pac NW coast, moving to north
Calif/southwest Oregon by Thu. As it does, it will maintain rather
baggy upper trough over the region, with main trough axis just
offshore. As such, look to remain coolish with showers. But, ,think
system may stay far enough offshore that will have some dry periods
mixed in that timeframe for Tue to Thu. Some differences with
long range models, as GFS holds more southerly flow aloft
(better for showers), but ECMWF being more southwesterly
(favoring less showers and GFS). Either way, still coolish as
will see plenty of clouds about the area. Does appear that the
whole upper trough will push inland Fri/Sat, with more showers.
/Rockey
&&
.AVIATION...Marine stratus continues to sit along the coast, and
as of 21z Wed, KAST has already dropped to MVFR ceilings, with
most other terminals sitting somewhere between 3500-6000 feet.
KONP is just barely at VFR thresholds, and sees a 60-70% chance of
ceilings dropping to MVFR within an hour or two (23z-00z Wed/Thu).
Once MVFR conditions develop at the coast, there is an additional
60-70% chance of IFR conditions developing later, around 03-06z
Thu.
At inland terminals, VFR conditions continue until later tonight,
when a more major front moves in around 11-13z Thu. All inland
terminals can expect around a 80-90% chance of MVFR ceilings and
rain. Chances of IFR ceilings at inland terminals is only around
10-20%, so have not included in the TAF.
Notes: The ASOS at KTTD is only available via the dial in option.
The AWOS at KMMV is still out of commission and there is still
no estimated time of repair.
PDX AND APPROACHES...VFR thresholds remain in place until Thursday
morning, when a front arrives and lowers ceilings to high end
MVFR. 80% confidence in MVFR ceilings by 15z Thu, but only 10%
confidence in IFR ceilings. Winds become southerly and gust up to
around 10-12 kts after the front passes. /JLiu
&&
.MARINE...A warm front associated with a low well off the B.C.
coast moves across the waters this evening. The following cold
front moves across the waters Thu afternoon. Expect to see gusts
25 to 30 kt across the waters in the afternoon, with a brief
period with gusts to 35 kt for zones PZZ272,273,252,253 as the
front moves through the waters. High resolution guidance also
suggests a 30-50% chance of occasional gale force gusts to around
34 kt the frontal passage between 12-4 PM PDT Thursday. Wind
forecast is a bit higher then previous forecast Thursday night so
will extend the Small Craft Advisory into Friday evening. In
addition to the wind seas will be steep and choppy.
Looking ahead into the weekend and early next week, marine conditions
will be characterized by a series of relatively weak frontal systems.
NBM guidance suggests a less than 5% chance of wind gusts exceeding
34 kt across the waters in the long term forecast. /mh
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 5 AM to 8 PM PDT Thursday for PZZ210.
Small Craft Advisory from 5 AM Thursday to 2 AM PDT Saturday for
PZZ251>253-271>273.
&&
$$
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